In the pharmaceutical industry, it is important to know the parameters which characterize the tableting properties of powders which are compressed to form tablets used for human and animal consumption. It is desirable to have a method of quantitatively evaluating the tableting characteristics of a powder before it is mixed with other materials so as to permit detection and rejection of lots that, if used, would introduce problems such as capping or excessive friability. Such methods should also be useful to assist in formulating new chemical entities. By being able to initially define what tableting indices are critical, and then by being able to experimentally measure such indices based on compacts formed from test samples, the need for formulation of large samples and the control and predictability of the tableting performance can be much more predictability controlled and achieved.
It is believed that the bonding index of the powder is probably the most important parameter, and it has been further determined that the bonding index itself involves two tableting indices, one being referred to as the best case bonding index, and the other being referred to the worst case index. The difference between the best case and worst case bonding indices indicate whether the material is viscoelastic. If the final formulation of the powder is viscoelastic, then it will be sensitive to the rate that the tableting machine is run, that is, to the strain rate. However, viscoelasticity promotes stronger bonding. Thus, a quantitative evaluation of the difference between these best case and worst case bonding indices is highly desirable.
An evaluation of the bonding index involves application of a compressive load to a surface of a compact (i.e., a rather large blocklike tablet of compressed powder). This involves creation of a partial spherical indentation into the surface of the compact to permit a hardness value to be determined.
To experimentally determine the worst case bonding index, it is typical to use a procedure whereby a steel sphere or ball is bounced off a surface of the compact, whereby the steel ball creates a partial spherical depression in the compact. The steel ball, upon impact against the surface of the compact, imposes a compressive load on the compact, and the mean pressure under the ball is an indication of the shear strength of the compact under compressive load. This hence permits determination of the worst case bonding index. A desirable pendulum-type impact tester for this purpose is illustrated and described in co-pending application Ser. No. 07/197,841, filed on May 24, 1988, now U.S. Pat. No. 4,885,933.
To obtain a hardness which corresponds to the best case bonding index, however, requires use of a procedure which creates a very slow effective strain rate when a compressive load is imposed on the compact, such as by means of a sphere. Hence, the present invention relates to an improved apparatus which is designed specifically for permitting slow strain rate shear strength testing of a compact formed of pharmaceutical powders or particulate materials, which apparatus permits a hardness determination by using a very slow effective strain rate so as to permit determination of the best case bonding index of the material.
More specifically, this invention relates to an improved slow strain rate shear strength tester for a compact, as aforesaid, which tester involves a ram structure which is capable of pushing a ball into the surface of the compact while allowing a significant dwell time during the compression to obtain a partially or a completely relaxed hardness value due to viscoelastic effects.
In the improved tester of this invention, the apparatus is provided with an improved compact or tablet holding head arrangement which can be readily mounted on or removed from the apparatus, and which head arrangement itself greatly facilitates the mounting on or removal of a compact when testing is to be carried out.
This improved tester, as aforesaid, in addition readily mounts thereon an alternate compact-supporting head arrangement which permits experimental determination of a slow strain rate tensile strength measurement of a compact, which tensile strength measurement and slow strain rate hardness measurement are used for determining the best case bonding index. This tensile strength measurement is also used for determining the worst case bonding index.
Other objects and advantages of the apparatus according to the present invention will be apparent to persons familiar with arrangements of this general type upon reading the following specification and inspecting the accompanying drawings.